Tuberculosis still remains a health problem in the United States; more than 30,000 newly diagnosed cases of tuberculous disease were reported in 1977. It is estimated that there are more than 87,000 patients with this disease at any time. The elderly population is more susceptible to this disease. Adverse reactions to anti-tuberculous drugs are more frequent in the elderly since the dose sizes are based on pharmacokinetic data in young adults and drug-drug interactions are not known. The present proposal seeks to obtain pharmacokinetic parameters (plasma half-life, peak and plateau levels of total and free drug in plasma, area under the plasma level-time curve, urinary excretion and clearance of drug and important metabolites) after daily doses of anti-tuberculous drugs. Three drugs, isoniazid, ethambutol and rifampicin, will be studied in a group of young (18-35 yr) and a group of older (greater than or equal to 60 yr) tuberculous patients. The pharmacokinetic data in the two groups of patients will be compared and correlations, if any, will be obtained between age and plasma half-life of the drug, age and urinary clearance, age and apparent volume of distribution and creatinine clearance and drug clearance. The effect of one drug on the pharmacokinetics of the other will be studied. Relationships between any incidental toxicity and plasma levels and/or urinary clearance of the free drug (or important metabolites) will be established. The information obtained from the study will be important in establishing guidelines for optimum dosing and scheduling of antituberculous drugs, given in combinations, especially in the elderly.